Modulation



March 1, 1949. R. A. HENDERSON MODULAT I ON Filed May 10, 1945 IN V EN TOR. gay A. HF/YDE/Pm/V BY Patented Mar. 1, 1949 MODULATION Roy A. Henderson, West Collingswood, N. J assignor to Radio Corporation of America, a corporation of Delaware Application May 10, 1945, Serial No. 592,957

7 Claims. I

This application concerns modulation and in particular high efiiciency linear modulation of the type wherein a power stage has its grid grounded and its anode modulated.

Grounded grid amplifiers are known in the prior art, as are anode and grid modulators and modulators of the type wherein the modulating potentials are applied to the cathode. In these systems, however, when a high percentage of modulation is necessary distortion in many cases results.

An object of my invention is full modulation of the amplitude of carrier energy with little or no distortion resulting in the modulation.

This object is attained by providing an improved means of modulating a grounded grid power amplifier stage and also modulating the stage used to excite the power amplifier, in this manner obtaining a high degree of modulation with low distortion.

A further object of my invention is to provide a simple and eificient modulation circuit wherein .the power amplifier has its grid grounded and its cathode operating at high radio frequency potential, and wherein the exciter stage includes a tube having a screen grid and supplies excitation voltage to the cathode of the amplifier stage, and wherein for the sake of a high degree of linear modulation the anode of the amplifier stage and the anode and screen grid of the exciter stage are all modulated by inphase modulating potentials. Thus complete linear modulation is obtained if desired.

In describing my invention reference Will be made to the attached drawing wherein the single figure illustrates an embodiment of my improved simplified modulation system by means of which a high degree of modulation can be obtained without producing material distortion.

In the figure E designates a radio frequency exciter tube for the power amplifier stage A. The tube E may in turn be excited by carrier oscillations from a source in rectangle Iii. This source may comprise an oscillation generator and/or amplifiers and/or multipliers, if desired. The stage E may be considered the final stage in. the source l0, so that E and I8 are the exciters for the power amplifier stage A. The grid electrode of E is coupled in any approved fashion to the stage l0, and is biased as desired for example by means of a resistance M. The anode electrode l6 of tube E is connected by a radio frequency choke RFC to a positive potential source and coupled by condenser BC to a tuned circuit F sharply parallel tuned to the frequency of the radio frequency energy amplified by the tube E. This condenser coupling the anode Iii to F also serves as a direct current blocking condenser.

The voltage of radio frequency developed in the circuit F is fed to the cathodes I l of the power amplifier stage A so that this stage is cathode driven. The grid I8 of the tube of stage A is coupled to ground by a radio frequency bypassing condenser B and to a biasing source C through a resistance 20. The anode 22 of the tube A is coupled by a direct current blocking condenser 24 to the parallel tuned circuit C, which as illustrated includes a conventional coil and variable condenser with a point on the coil variably tapped to the load impedance which may be,as shown, an antenna. The anode 22 of tube A is connected by radio frequency choke 25 to a direct current plate supply source. A modulation transformer winding described hereinafter is in this connection.

Since the cathode ll of the power amplifier stage runs at high radio frequency potential the filament of the power amplifier tube is heated by a transformer D having a shielded secondary winding with a low capacity to ground. Radio frequency excitation energy is fed to the oathode, by way of the common cathode conductor, from a voltage stepped down point on the parallel tuned circuit F. The secondary winding on the transformer, (i. e., outer cathode leads) is shunted by two radio frequency coupling condensers G and H, to keep the cathodes of the power amplifier tube at essentially the same radio frequency potential.

The modulator stage I may comprise a pair of tubes 32 and 34 with their control grids excited differentially by modulation currents or voltages from a modulation source 30. The anodes 32 and 34 of the tubes of the modulator stage are coupled differentially by the primary winding #1 of a transformer J. This transformer J has two secondary windings #2 and #3. Secondary winding #2 carries the direct current plate voltage for the anode 22 of the power amplifier A and for the anode I6 of the exciter tube E. A potential dropping resistance K may be included in this last direct current path if the tubes are such that anode l6 operates better at lower direct current potential. The winding #2 also supplies modulation potential superimposed on the direct current potentials supplied to the anodes I6 and 22 of the tubes E and A. The #3 winding of this transformer supplies direct current and modulation potentials superimposed thereon to the screening electrode l 5 of the tube E. The voltages generated in the #2 and #3 windings are in phase, and are proportional to the direct current plate and screen voltages supplied to the respective electrodes of the tubes E and A. Therefore, it is evident that modulation voltages will be applied to the plates of the power amplifier and exciter and to the screen grid of the exciter. The modulation may be increased to 100% without undue distortion. Why distortion is low at full modulation may be explained as follows:

The anode' 16 receives plate voltage from the same source that provides plate voltage for anode 22. Likewise, both anodes receive modulation from the same winding 2 of the modulation transformer. At 100% modulation the peak value of the modulating voltage will equal the direct current plate voltage.

Likewise, winding 3 of the modulation transformer is so proportioned that (at 100% modulation) the peak value of the modulating voltage equals the direct current screen voltage of l5.

Since the screen l5 and anode l6 are modulated proportionally conditions are ideal for low distortion in the exciter stage. Since the exciter stage and power amplifier stages are modulated together the power to the antenna (which is provided by both the power amplifier and the exciter) is nearly modulated. Therefore the conditions are ideal for low distortion.

What is claimed is:

1. In a signalling system, a power amplifier stage including a tube having output electrodes including an anode and having input electrodes, an exciter stage tube having a plurality of electrodes including a control electrode and having a cathode and an anode in a circuit wherein radio frequency voltage or current flows, a coupling between said circuit and the input electrodes of said first named tube to supply radio frequency energy to the said input electrodes of said first named tube, a source of modulating potentials,

and a modulator stage coupling said source ofmodulating potentials to the control electrode of said exciter stage tube and to the anode electrodes of both of said tubes, the coupling being such that said modulating potentials are of substantially like phase on the said electrodes.

2. In a signalling system, a power amplifier stage including a tube having output electrodes including an anode and having input electrodes including a cathode and a grid operating at ground or equivalent radio frequency potential, an exciter stage tube having electrodes including a screen grid and having a cathode and an anode in a circuit wherein radio frequency voltages appear, a coupling between said circuit and the input electrodes of said first tube to supply radio frequency energy to the said input electrodes of said first tube, a source of modulating potentials, and a modulator stage coupling said source of modulating potentials tothe screen grid electrode of said second mentioned tube and to the anode electrodes of both of said tubes, the coupling being such that said modulating potentials are of substantially like phase on the said electrodes.

3. In a signalling system, a power amplifier stage including a tube having output electrodes including a cathode and an anode and having input electrodes including said cathode, an exciter stage tube having a cathode and a plurality of control electrodes oneof which is excited, with respect to said last named cathode, by radio frequency voltage or current to be amplified and modulated, said last named tube having output electrodes including said last named cathode and an anode which supplies amplified radio frequency energy to the input electrodes of said first tube, a source of modulating potentials, a modulator stage coupled thereto and transformer means coupling said modulator stage to another control electrode of said exciter stage tube and to the anode electrodes of both of said tubes, the arrangement being such that said modulating potentials are of substantially like phase on the said electrodes.

4. In a signalling system, a power amplifier stage including a tube with an anode electrode and a control grid operated at ground potential and a cathode operated above ground radio frequency potential, an exciter stage tube having a cathode anda plurality of control electrodes one of which is excited, with respect to said last named cathode, by radio frequency voltage or current to be amplified and modulated, said last tube having output electrodes which include said last named cathode and which supply amplified radio frequency energy to the cathode of said first tube, a source of modulating potentials, a modulator stage coupled thereto and a transformer coupling said modulator stage to another control electrode of said exciter stage tube and to the anode electrodes of both of said tubes, said transformer coupling being such that said modulating potentials are of substantially like phase on the said electrodes.

5. In a signalling system, a power amplifier stageincluding a tube with an anode electrode and a control grid operated at ground potential and a cathode operated above ground radio frequency potential, an exciter stage tube having a cathode, an anode and a plurality of control electrodes one of which is excited by, with respect to said last cathode, radio frequency voltage or current to be amplified and modulated, a parallel tuned circuit coupling the anode of said last named tube to the cathode of the first tube to supply amplified radio frequency energy to the cathode of said first tube, a source of modulating potentials, a modulator stage and a transformer coupling said source of modulating potentials to another control electrode of said exciter stage tube and to the anode electrodes of both of said tubes, said coupling being such that said modulating potentials are of substantially like phase on all of the said electrodes.

6. In a signalling system, a power amplifier tube having a cathode heated by a transformer shielded with respect to ground radio frequency potential, a control grid operated at ground radio frequency potential and an anode coupled to an output circuit, a radio frequency exciter tube having a cathode, an anode, a screen grid and a control electrode coupled to a source of radio frequency voltage or current to be amplified and modulated, a voltage step down coupling from the anode of the exciter tube to the cathode of the power amplifier tube, a source of modulating potentials, and a modulator tube coupling said source of modulating potentials to the screen grid of said exciter tube and to the anode electrodes of both of said tubes to apply modulating potentials of substantially like phase to said electrodes.

7. In a signalling system, a power amplifier tube having a cathode operating above ground radio frequency potential, a control grid operated at ground radio frequency potential and an anode coupled to an output circuit, a radio frequency exciter tube having a cathode, an anode, a screen grid and a control electrodecoupled to a source of radio frequency voltage or current to be am- ROY A. HENDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

5 UNITED STATES PATENTS Number Name Date 2,130,893 Mouromtseilf et al. Sept. 20, 1938 2,159,020 Ferrell May 23, 1939 16 OTHER REFERENCES The Radio Amateurs Handbook-1941. 

